This invention relates to an exposure method, an exposure apparatus and a device manufacturing method. More particularly, the invention concerns an exposure method and apparatus for transferring a very fine circuit pattern onto a photosensitive substrate through multiple exposures. The exposure method and apparatus of the present invention are suitably usable for manufacture of various devices such as semiconductor chips (e.g., ICs or LSIs), display devices (e.g., liquid crystal panels), detecting devices (e.g., magnetic heads), or image pickup devices (e.g., CCDs), or for production of patterns to be used in micro-mechanics.
The manufacture of microdevices such as ICs, LSIs or liquid crystal panels, for example, use a projection exposure method and a projection exposure apparatus wherein a circuit pattern formed on a photomask or reticle (hereinafter, xe2x80x9cmaskxe2x80x9d) is projected through a projection optical system onto a photosensitive substrate such as a silicon wafer or a glass plate (hereinafter, xe2x80x9cwaferxe2x80x9d) which is coated with a photoresist, for example, by which the circuit pattern is transferred (photoprinted) to the wafer.
In order to meet enlargement of integration of a device (chip), miniaturization of a pattern to be transferred to a wafer, that is, improvements in resolution, as well as enlargement in area of each chip have been desired. Thus, in a projection exposure method and projection exposure apparatus which plays a main role in wafer microprocessing procedure, many attempts have been made to improve the resolution and to enlarge the exposure area in order that an image of a size (linewidth) of 0.5 micron or less can be formed in a wider range.
FIG. 1 is a schematic view of a conventional projection exposure apparatus, wherein denoted at 191 is an excimer laser which is a deep ultraviolet light exposure light source. Denoted at 192 is an illumination optical system, and denoted at 193 is illumination light. Denoted at 194 is a mask, and denoted at 195 is object side exposure light emitted from the mask 194 and entering an optical system 196 which is a reduction projection optical system. Denoted at 197 is image side exposure light emitted from the optical system 196 and impinging on a substrate 198 which is a photosensitive substrate (wafer). Denoted at 199 is a substrate stage for holding the photosensitive substrate.
Laser light emitted from the excimer laser 191 is directed by a guiding optical system to the illumination optical system 192, by which the laser light is adjusted to provide the illumination light 193 having a predetermined light intensity distribution, a predetermined orientation distribution, and a predetermined opening angle (numerical aperture NA), for example. The illumination light 193 then illuminates the mask 194.
The mask 194 has formed thereon a pattern of a size corresponding to the size of a fine pattern to be formed on the wafer 198 but as being multiplied by an inverse of the projection magnification of the projection optical system 196 (namely, 2x, 4x or 5x, for example). The pattern is made of chromium, for example, and it is formed on a quartz substrate. The illumination light 193 is transmissively diffracted by the fine pattern of the mask 194, whereby the object side exposure light 195 is provided. The projection optical system 196 serves to convert the object side exposure light 195 to the image side exposure light 197 with which the fine pattern of the mask 194 can be imaged upon the wafer 198 at the projection magnification and with a sufficiently small aberration. As shown in a bottom enlarged view portion of FIG. 1, the image side exposure light 197 is converged on the wafer 198 with a predetermined numerical aperture NA (=sin xcex8), whereby an image of the fine pattern is formed on the wafer 198. The substrate stage 199 is movable stepwise along the image plane of the projection optical system to change the wafer 198 position relative to the projection optical system 196, such that fine patterns are formed sequentially on different regions on the wafer 198 (e.g., shot regions each covering one or more chips).
However, with projection exposure apparatuses currently used prevalently and having an excimer laser as a light source, it is still difficult to produce a pattern image of 0.15 micron or less.
As regards the resolution of the projection optical system 196, there is a limitation due to a xe2x80x9ctrade offxe2x80x9d between the depth of focus and the optical resolution attributable to the exposure wavelength (used for the exposure process). The resolution R of a pattern to be resolved and the depth of focus DOF of a projection exposure apparatus can be expressed by Rayleigh""s equation, such as equation (1) and (2) below.
R=k1(xcex/NA)xe2x80x83xe2x80x83(1)
DOF=k2(xcex/NA2)xe2x80x83xe2x80x83(2)
where xcex is the exposure wavelength, NA is the image side numerical aperture which represents the brightness of the projection optical system 196, and k1 and k2 are constants which are determined by the development process characteristics, for example, and which are normally about 5-0.7. From equations (1) and (2), it is seen that, while enhancement of resolution, that is, making the resolution R smaller, may be accomplished by enlarging the numerical aperture NA (NA enlarging), since in a practical exposure process the depth of focus DOF of the projection optical system 196 can not be shortened beyond a certain value, increasing the numerical aperture NA over a large extent is not attainable, and also that, for enhancement of resolution, narrowing the exposure wavelength xcex (band-narrowing) is any way necessary.
However, such band-narrowing encounters a critical problem. That is, there will be no glass material available for lenses of the projection optical system 196. In most glass materials, the transmission factor is close to zero, in respect to the deep ultraviolet region. Although there is fused silica which is a glass material produced for use in an exposure apparatus (exposure wavelength of about 248 nm) in accordance with a special method, even the transmission factor of fused silica largely decreases in respect to the exposure wavelength not longer than 193 nm. It is very difficult to develop a practical glass material for a region of an exposure wavelength of 150 nm or shorter, corresponding to a very fine pattern of 0.15 micron or less. Further, glass materials to be used in the deep ultraviolet region should satisfy various conditions, other than the transmission factor, such as durability, uniformness of refractive index, optical distortion, easiness in processing, etc. In these situations, the availability of practical glass materials is not large.
As described, in conventional projection exposure methods and projection exposure apparatuses, the band-narrowing of exposure wavelength to about 150 nm or shorter is required for formation of a pattern of 0.15 micron or less upon a wafer 198 whereas there is no practical glass material for such wavelength region. It is therefore very difficult to produce a pattern of 0.15 micron or less on a wafer.
Recently, an exposure method and apparatus for performing a dual exposure process, comprising a periodic pattern exposure and a standard (ordinary) exposure, to a substrate (photosensitive substrate) to be exposed, has been proposed in an attempt to producing a circuit pattern including a portion of 0.15 micron or less.
Here, the term xe2x80x9cstandard exposurexe2x80x9d or xe2x80x9cordinary exposurexe2x80x9d refers to an exposure process by which an arbitrary pattern can be photoprinted although the resolution is lower than that of the periodic pattern exposure. A representative example of it is the exposure process to be performed by projection of a mask pattern with a projection optical system.
A pattern to be printed by the standard exposure (hereinafter, xe2x80x9cstandard exposure patternxe2x80x9d) may include a very fine pattern less than the resolution. The periodic pattern exposure is a process for forming a periodic pattern of a similar linewidth as that of the very fine pattern.
Such periodic pattern exposure may use a Levenson type phase shift mask, for example. An example of a dual exposure process is shown in FIGS. 2A-2C. A periodic pattern (FIG. 2A) and a standard exposure pattern (FIG. 2B) are printed on the same position, by which a very fine pattern (FIG. 2C) corresponding to a composite image of them is produced.
In this manner, a pattern to be produced finally is photoprinted as a standard exposure pattern, but, since the standard exposure pattern contains a pattern portion smaller than the resolution, a periodic pattern of high resolution is printed there. By this, the resolution of the standard exposure pattern can be improved and, finally, a desired pattern including a very fine line smaller than the resolution can be produced.
In the dual exposure process, in order to improve the resolution of a standard exposure pattern (FIG. 2B), a high resolution periodic pattern (FIG. 2A) is printed on the same position. In such dual exposure process, if the elongation direction of the fine-line portion of the pattern of FIG. 2B is registered with the periodicity direction in FIG. 2A, no particular problem arises.
If however a standard exposure pattern includes fine lines different directions, such as shown in FIG. 3B wherein there are fine lines extending in the same direction as the periodicity and fine lines extending in a direction perpendicular thereto, while the fine lines in the same direction as the periodicity may be resolved, the fine lines extending perpendicularly to the periodicity may not be resolved.
Details will be described with reference to a pattern called a gate pattern or a T gate pattern, used with a positive type resist material, in conjunction with FIGS. 2A-2C and 3A-3C. It is assumed now that in these drawings the periodic pattern comprises such pattern that light passes therethrough by which its phase is inverted. This periodic pattern has a periodicity not less than 2. The standard exposure pattern comprises such pattern that light passes through the peripheral portion around the pattern which blocks light, and it has a binary amplitude with constant phase.
For example, in FIGS. 2A-2C, each fine line of the gate pattern of FIG. 2B (standard exposure pattern) is oriented in the same direction as the periodic pattern of FIG. 2A. Thus, the resolution of fine line of the gate pattern of FIG. 2A (standard exposure pattern) can be increased.
In the example of a T gate pattern shown in FIG. 3B, there are additional fine lines extending, like T-shape, orthogonally to fine lines of a gate pattern. Thus, there are fine lines extending in different directions.
If there are fine lines extending longitudinally and laterally, resolution is particularly difficult to achieve in such zone (hard-resolution zone) where a fine line and a pattern are juxtaposed with each other with a spacing not larger than the resolution. In order to attain improved resolution for such zone, use of a periodic pattern such as shown in FIG. 3A is necessary. However, mere use of such periodic pattern would not result in successful resolution of fine lines extending in a direction perpendicular to the periodicity, although resolution may be accomplished for the hard-resolution zone.
Therefore, when a dual exposure process using a periodic pattern and a standard exposure pattern is to be performed, a pattern to be produced finally is limited in some cases, depending on the orientation of the periodic pattern used. Particularly, as regards a pattern having fine lines extending in a direction different from the periodicity direction of the periodic pattern, it is difficult to well meet the same, with the dual exposure process used conventionally. For example, at the pattern spacings xe2x80x9cAxe2x80x9d, adjacent patterns may become continuous and they may not be separated sharply.
It is accordingly an object of the present invention to provide an exposure method, an exposure apparatus and/or a device manufacturing method, by which, even where a standard exposure pattern contains fine lines extending in different directions, a desired pattern can be produced finally in a dual exposure process.
In accordance with an aspect of the present invention, there is provided an exposure method for exposing a substrate through a multiple exposure process including a first exposure using a first pattern having fine line elements of different directions, and a second exposure using a second pattern including a periodic pattern: wherein a periodicity direction of the periodic pattern is registered with a direction along which fine line elements of a predetermined direction, of the different directions, are arrayed, while, at least in a portion of the periodic pattern, a pattern or a boundary between adjacent patterns as well as a portion of or the whole of the fine line elements of the particular direction are adapted to be printed at the same location; and wherein the second pattern is so structured that one or those of the fine line elements of the first pattern extending in a particular direction different from the predetermined direction are not superposed with the periodic pattern.
In accordance with another aspect of the present invention, there is provided an exposure method for exposing a substrate through a multiple exposure process including a first exposure using a first pattern having fine line elements of different directions, and a second exposure using a second pattern including a periodic pattern: wherein a periodicity direction of the periodic pattern is registered with a direction along which fine line elements of a predetermined direction, of the different directions, are arrayed, while, at least in a portion of the periodic pattern, a light blocking area or a boundary of a phase serviceable as a light blocking area as well as a portion of or the whole of the fine line elements of the particular direction are adapted to be printed at the same location; and wherein the second pattern is so structured that one or those of the fine line elements of the first pattern extending in a particular direction different from the predetermined direction are not superposed with the periodic pattern.
The periodicity direction of the periodic pattern may be registered with a direction along which most fine line elements are arrayed.
The particular direction different from the predetermined direction may correspond to the periodicity direction.
The periodic pattern may comprise a periodic pattern having a periodicity not less than 2 and being provided by one of a Levenson type phase shift mask, an edge type phase shift mask and a binary type mask.
The second pattern may have a region where no periodic pattern is formed, and an isolated line element may be formed in that region so that the isolated line element is to be superposed with a fine line element of a direction different from the predetermined direction.
The periodic pattern and the isolated line element may be defined by one of a light blocking area and a light transmitting area.
The isolated line element may have a size or a shape which is different in accordance with a fine line element of a direction different from the predetermined direction.
Those of the fine line elements of a direction different from the predetermined direction may include at least one having a linewidth larger than a resolution.
In accordance with a further aspect of the present invention, there is provided an exposure method for exposing a substrate through a multiple exposure process including a first exposure using a first pattern and a second exposure using a second pattern: wherein the first pattern includes a periodic pattern and has a region for execution of correction of pattern distortion due to an optical proximity effect in the exposure of the second pattern.
The region for execution of the correction of pattern distortion due to the optical proximity effect may be provided by a region inside a pattern region for the periodic pattern of the first pattern where no periodic structure is formed.
The second pattern may be adapted to produce a light intensity distribution of multiple levels, upon a surface of the substrate.
The second pattern may have a shape directly corresponding to a design pattern to be produced on the substrate or similar to the design pattern.
The first pattern may be arranged so that the correction of pattern distortion due to the optical proximity effect is performed such that a light intensity distribution to be provided thereby may become similar to the design pattern.
The first and second patterns may be arranged so that the correction of pattern distortion due to the optical proximity effect is performed such that a light intensity distribution of a composite image to be produced by superposed exposures of the first and second patterns, becomes similar to the design pattern.
In the region inside the periodic pattern region of the first pattern where there is no periodic structure, a region having a locally thickened linewidth may be defined by which the correction of pattern distortion due to the optical proximity effect is performed.
In the region inside the periodic pattern region of the first pattern where there is no periodic structure, and in such portion where a fine line element of the second pattern, of a direction not orthogonal to a periodicity direction of the first pattern, there may be an isolated line element formed, and a linewidth of the isolated line element may be made larger than the linewidth of the fine line element of the second pattern.
The linewidth of the isolated line element may be optimized so that a fine line element of a composite image to be produced by the periodic pattern of the first pattern and a fine line element of a composite image to be produced by the isolate line element have substantially the same linewidth.
A correction pattern may be formed in a region inside the periodic pattern region of the first pattern where no periodic structure is formed, by which the correction of pattern distortion due to the optical proximity effect is performed.
When an isolated line element to be superposed with a fine line element of the second pattern, of a direction not orthogonal to the periodicity direction of the first pattern, is present in the region inside the periodic pattern region of the first pattern where no periodic structure is formed, the correction pattern may be formed to substantially correct contraction of the isolated line element.
In the first pattern, a pattern having a linewidth at least three times larger than the narrowest line element of the second pattern may be formed with a light blocking portion, for light quantity adjustment.
In the first pattern, the linewidth may be adjusted to adjust the whole light quantity balance of the first pattern.
The first pattern may include intersecting patterns, and a light blocking portion may be defined at or adjacent an intersection of the intersecting patterns.
The first pattern may include L-shaped orthogonal patterns, and a light blocking portion may be defined at or adjacent an intersection of the orthogonal patterns.
The first pattern may include a T-shaped orthogonal patterns, and a light blocking portion may be defined at or adjacent an intersection of the orthogonal patterns.
The first pattern may comprise a periodic pattern having a periodicity not less than 2 and being provided by one of a Levenson type phase shift mask and a binary type mask.
In accordance with a still further aspect of the present invention, there is provided an exposure method for exposing a substrate through a multiple exposure process including a first exposure using a first pattern having a fine line and a second exposure using a second pattern having a periodic pattern: wherein a length of a predetermined light passing region of the periodic pattern is made shorter than another light passing region of the periodic pattern, to thereby suppress distortion in a predetermined portion of the first pattern ranging to the predetermined light passing region in the multiple exposure process.
In accordance with a yet further aspect of the present invention, there is provided an exposure method for exposing a substrate through a multiple exposure process including a first exposure using a first pattern having a fine line and a second exposure using a second pattern having a periodic pattern: wherein a length of a predetermined light passing pattern of the periodic pattern is set so that an exposure amount distribution at an edge of the predetermined light passing pattern of the periodic pattern has a tilt opposite to that of an exposure amount distribution at an edge of a predetermined portion of the first pattern, ranging to the predetermined light passing pattern, such that they are combined with each other during the multiple exposure process.
In accordance with a still further aspect of the present invention, there is provided an exposure method for exposing a substrate through a multiple exposure process including a first exposure using a first pattern having a fine line and a second exposure using a second pattern having a periodic pattern: wherein the second pattern is arranged so as to suppress distortion in a predetermined portion of the first pattern during the multiple exposure process.
The length of the periodic pattern may be adjusted such that a length of a periodic pattern upon the substrate in its lengthwise direction is made equal to the length of the first pattern in the same direction as the lengthwise direction of the periodic pattern.
The length of the periodic pattern upon the substrate may be set within an extent from the length equal to a length of a fine line element of the first pattern in the same direction as the lengthwise direction of the periodic pattern, to a length as determined by subtracting, from the length of the fine line element, a pattern width of of a pattern portion of the first pattern, ranging from the periodic pattern.
As regards the length of the periodic pattern upon the substrate, a length corresponding to the periodic pattern length plus a length corresponding to contraction of the periodic pattern may be accumulated.
The periodic pattern may comprise a periodic pattern having a periodicity not less than 2 and being provided by one of a Levenson type phase shift mask, an edge type phase shift mask and a binary type mask.
In accordance with a yet further aspect of the present invention, there is provided a mask usable with an exposure method such as recited above, for supplying the first pattern in that exposure method.
In accordance with a yet further aspect of the present invention, there is provided a mask usable with an exposure method as recited above, for supplying the second pattern in that exposure method.
In accordance with a yet further aspect of the present invention, there is provided a device manufacturing method including a process for producing a device by use of an exposure method as recited above.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.